Geosynthetics October/November 2023 - 20

Development of cation exchange processes in geosynthetic clay liners
The findings from this
investigation improve
understanding of
the fundamental
behavior of GCLs
in the presence of
chemical solutions.
Conclusions
This investigation was conducted to systematically
analyze the cation exchange
characteristics of a conventional GCL
as a function of time and concentration
of hydration fluids at 20°C (68°F). Tests
were conducted on 150 x 150 mm (5.9 x
5.9 inch) square specimens of the GCL,
which was a medium-weight needlepunched
nonwoven-nonwoven GCL
product with granular bentonite. The
GCL specimens were hydrated over eight
conditioning periods: 4 hours, 8 hours, 1
day, 2 days, 4 days, 8 days, 16 days and
32 days in four hydration solutions: DI
water and 2 mM, 50 mM and 200 mM
CaCl2 solutions.
The average cation exchange capacity
of the GCL specimens was determined
to be 70.5 cmol+/kg. The concentration
of hydration fluid and conditioning time
affected cation exchange behavior of
the GCL. For the 50 mM and 200 mM
CaCl2 solutions, variations were significant,
where the amount of bound Ca2+ in
the bentonite increased rapidly and the
amount of bound Na+ in the bentonite
decreased rapidly, with negligible variations
over time for the DI water and 2
mM CaCl2 solutions. The majority of
the exchange processes was completed
within days (approximately less than 8
days) at these relatively high calcium
chloride solution hydration/exposure
conditions. Relatively low changes in
bound K+ and Mg2+ concentrations were
observed. Electrical conductivity of the
hydration solutions was not indicative
of the physicochemical processes occurring
in the GCLs. The timing of cation
exchange processes is critical for various
applications ranging from laboratory
and field testing for fluid transport
characteristics of GCLs to maintaining
integrity of GCL characteristics in
field applications with timely cover of
constructed liners, potential prehydration
and/or first/repeated exposure to
20
Geosynthetics | October November 2023
contained liquids. This test program
provides worst-case scenario conditions
for timing of exchange processes.
The timing of exchange processes may
be different under different exposure
conditions such as different hydration
solutions and hydration over/under/
between different variably saturated
soils. The findings from this investigation
improve understanding of the
fundamental behavior of GCLs in the
presence of chemical solutions.
Acknowledgments
This investigation was supported by a grant
from the National Science Foundation
(ROA Supplement to CMMI-1812550),
CETCO/Mineral Technologies Inc. and
the Global Waste Research Institute.
References
ASTM D5887. (2022). " Standard Test Method for
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Geosynthetics October/November 2023

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